A comprehensive analysis of the specific characteristics of a temporomandibular joint, also known as a temporomaxillary joint, as compared to the other diarthroses of the human body, is made in French patent application FR 2,558,721 in the name of the company Landos—Application Orthopédique Francaise, published on Aug. 2, 1985, in the preamble to the description of a prosthesis for a joint of this type.
The temporomandibular joint (abbreviated TMJ) is delimited by a cranial articular surface underlying the temporal bone and by an articular surface overlying the mandible. The glenoid fossa of the TMJ is pellucid since the bony layer that isolates the fossa from the overlying cranial mass is between several tenths of a millimeter and a maximum of two millimeters thick. The mandibular articular surface is constituted by the condyle, which surmounts the ascending branch of the mandible.
Between the temporal and mandibular articular surfaces is a fibrocartilaginous meniscus that covers the mandibular condyle and is continuous with the outer pterygoid muscle so that the meniscus is partly responsible for the movements that open the mouth.
The dynamics of TMJs are very distinctive, and unlike “closed” joints like those of the elbow or the knee, the temporomandibular joint is an “open” joint. It is a joint that is “suspended” from the skull, both by its own means and by the elevator muscles.
Taking into account these anatomical constraints, the above-mentioned patent application proposes a TMJ prosthesis comprising a self-tapping threaded screw surmounted by a ball-shaped head that cooperates with the spherical cavity of a socket. The threaded screw is designed to be screwed into the ascending branch of the mandible, and the socket is placed in the glenoid fossa of the joint.
This prosthesis allows the complex movements of the natural joint, but its ball-and-socket-based structure is reminiscent of the techniques used for the joints of the long bones.
Thus, it does not seem particularly suited to a TMJ. Moreover, its implantation is, quite clearly, particularly invasive.
In order to minimize the surgical procedure as much as possible, and particularly to avoid ablating a usable part of the condyle, European patent application EP 0,337,749 in the name of the company Vitek, published on Oct. 18, 1989, teaches the use of a “mini” temporomandibular condyle prosthesis that cooperates with an adapted glenoid fossa prosthesis.
The condyle prosthesis described is right-angle shaped, one branch being laterally attached to the natural condylar neck. The other branch has a convex surface that defines the articular surface of the artificial condyle.
The implantation of this prosthesis does not require an incision below the angle of the jaw, but makes it necessary to form a shoulder capable of receiving the upper end of the natural condyle.
According to another method for reconstructing a TMJ, German patent application DE 3,605,776 in the name of D. Kubein-Meesenburg, published on Sep. 3, 1987, proposes a meniscal endoprosthesis.
What characterizes this prosthesis is that it is made of flexible plastic material and can be adapted to the shape of the TMJ.
In order to help maintain it in position, this prosthesis is in the shape of a disk wherein the top surface in contact with the fossa has a radius of curvature larger than the radius of curvature of the bottom surface in contact with the condyle.
However, the practitioner knows that it is impossible to attach this type of prosthesis to either the muscles or the ligaments, and thus, in addition to the fact that the implantation of a prosthesis made of plastic material is not recommended for more than three months, the durability of such a solution is doubtful, since the essential characteristic of the temporomandibular joint is that it is an open joint.
The upshot of the prior art described above is that there are many known types of TMJ endoprostheses, but that to date, there is no existing model that meets the precise needs of specialists in maxillofacial surgery, i.e. a prosthetic implant having a long life that can be implanted in a very non-invasive way.